1. Field of Invention
This invention concerns a method for determination of .sup.18 O/.sup.16 O and .sup.2 H/.sup.1 H ratios and .sup.3 H concentrations of xylem and subsurface waters using time series sampling, insulating sampling chambers, and combined .sup.18 O/.sup.16 O, .sup.2 H/.sup.1 H and .sup.3 H concentration data on transpired water. In particular, the method involves collecting water samples transpired from living plants and correcting the measured isotopic compositions of oxygen (.sup.18 O/.sup.16 O) and hydrogen (.sup.2 H/.sup.1 H and/or .sup.3 H concentrations) to account for evaporative isotopic fractionation in the leafy material of the plant.
2. Background Art and Related Art Disclosures
Traditional approaches to obtain samples of soil water or shallow groundwater for stable isotopic (.sup.18 O/.sup.16 O or .sup.2 H/.sup.1 H) or tritium (.sup.3 H) analyses have focused on intrusive sampling methods, including drilling wells, using hydropunches, digging trenches, or otherwise excavating the earth and obtaining water samples through pumping, using vacuum lysimeters, or removing the water from soils using physical methods such as compression, cryogenic trapping, or centrifugation, or chemical methods such as azeotropic distillation. Similarly, traditional approaches to obtain samples of plant xylem waters for isotopic characterization involve destructive sampling of woody plant material.
Many researchers have obtained water samples from various plant tissues for purposes of studying the isotopic compositions of the waters, as summarized in Stable Isotopes and Plant Carbon-Water Relations, 510 (1993), Academic Press.
Advances in Organic Geochemistry, 55-67 (1966), Pergamon Press, describes the use of plant xylem water to determine the stable isotopic compositions of hydrogen and oxygen in shallow environmental waters and shows that plant xylem water is not isotopically fractionated during root uptake or transport to the plant leaf as xylem water.
Isotopes and Radiation in Soil-Plant Nutrition Studies, IAEA:410-415 (1965) shows that bulk leaf water becomes enriched in the heavy isotopes of hydrogen and oxygen due to evaporative fractionation. Because the amount of heavy sotope enrichment in water extracted from the leafy tissues of plants depends on such factors as transpiration rate, humidity, wind velocity, plant species, and local soil conditions, it is difficult to accurately predict the degree of heavy isotope enrichment in the leafy material of a given plant.
It would, therefore, be advantageous to have available a method for determination of a degree of heavy isotope enrichment in the leafy material of plants.
Stable Isotopes in Plant Carbon-Water Relations: 529-540 (1993) presents theoretical aspects of stable isotope relationships in plant leaf tissue and demonstrates the difficulty in accurately modeling or predicting the stable isotopic compositions of leaf water due to compartmentalization, and perhaps to species-dominated and capacitance-related effects.
Stable Isotopes in Plant Carbon-Water Relations, Physical Ecology Series, Eds. Ehleringer et al., Academic Press, Inc., pages 71-90 (1993) discusses the theory and modeling of stable isotope relations in transpired water and shows that precise application of the existing models require stable isotopic analysis of plant stem water and atmospheric water vapor.
Transpired waters, collected and analyzed for the stable isotopic compositions of hydrogen and oxygen provided similar results. As described in Radiation and Environmental Biophysics, 11:41-52 (1974), transpired waters were shown to be isotopically disturbed, with non-equilibrium enrichments in the heavy isotopes of hydrogen and oxygen due to evaporation during water residence in the plant leaf.
Because mass dependent fractionation of light isotopes depends only on the relative masses of the fractionating isotopes, .sup.3 H fractionation in the leafy materials of plants is expected to be 50% stronger than .sup.2 H fractionation under equilibrium conditions, and 33%-50% stronger than .sup.2 H fractionation under disequilibrium conditions, as reported in Tritium in the Physical and Biological Sciences, 1, IAEA Report No. ST1/PUB/39: 161 (1962).
Environmental Tritium in Trees, IAEA-WM-232/44: 405-418 (1979) showed that one can determine the distribution of .sup.3 H in shallow subsurface waters by conducting .sup.3 H activity measurements of plant xylem waters.
To accurately determine the stable isotopic compositions of hydrogen and oxygen and the tritium concentrations of soil waters and shallow groundwaters using analytical data for transpired waters, it is necessary to determine the amount of evaporative fractionation that has occurred during water residence in the plant leaf. However, a method to do so is currently not available. Until the current invention, therefore, lack of knowledge regarding the degree to which tritium concentrations are disturbed during evaporation has made it impossible to accurately estimate tritium concentrations in subsurface waters from transpired water data.
It would be, thus, highly desirable to provide a sampling approach, analytical strategy, and data interpretation technique that would allow the determination of the amount of hydrogen and oxygen isotope fractionation in transpired water samples, and to use this information to determine the amount of evaporative tritium fractionation in those same transpired waters. This would allow the quantitative determination of the stable isotopic compositions and tritium activities of waters being used by the plant, whether those waters are from the saturated zone (shallow groundwaters) or the unsaturated zone (shallow soil waters).
It is, therefore, the primary object of the current invention to provide a method of sampling transpired waters, an analytical approach, and a data interpretation technique that would allow quantification of stable isotopic compositions and tritium activities of plant xylem waters and isotopically equivalent soil waters and/or shallow groundwaters being used by the plant.